Musannaf Ibn Abi Shayba 3:4295

Ali ibn Mushir narrated to us, from Ibn Abi Arubah, who said: I asked al-Hasan about a man who prays the obligatory prayer, then comes to the mosque while the people are praying that same prayer. He said: He should pray with them, except for these two prayers: the Fajr and the Asr.